Ion transport across cell membranes is essential to living cells. It is well known that certain proteins function as channels in cellular systems for polar and ionic small solutes, such as cations or anions, which are transported across phospholipid bilayers. Ionic Channels of Excitable Membranes, 2nd Edition, Sinauer, Sunderland, Mass. (1992). This conduction is characterized by both ion selectivity and unidirectionality of ion flow.
Ion channels are, in effect, selective pores that allow for the diffusion of ions across cellular membranes. For example, voltage-sensitivity calcium channels mediate the entry of calcium into many types of excitable cells and thus play an important role in neurotransmitter release and excitation-contraction (E-C) coupling. Potassium channels are a group of ubiquitously expressed proteins that serve numerous functions in both excitable and non-excitable cells. See http://rsb.info.nih.gov//Neurochem//BN5ht/MembTransp-BN5.html.
Anion permeability, particularly chloride permeability, is essential for volume, pH and membrane regulation in all cells. Presently, four major families of protein-based chloride channels are known. A growing number of human diseases are known to be caused by mutations in genes encoding these channels. For example, it is known that cystic fibrosis (CF) is caused by mutational defects in a gene encoding protein carriers for chloride channels. See Am J. Resp. Crit. Care Med., Vol. 163, No. 7, June 2001, 1683-1962. See also http://agreem.atsjournals.org/cgi/content/full/163/7/1683.
CF is a common and lethal autosomal recessive genetic disease caused by dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) membrane. In fact, absent or defective CFTR function alters both Na+ and CI− transport across multiple epithelia, which contributes to both the morbidity and mortality of the disease. Since discover of the ctfr gene more that 800 disease-causing alleles have been identified. See Am J. Resp. Crit. Care Med., id.
The genetics of CF are similar to that of other autosomal recessive diseases in that premature stop mutations within the gene are relatively common. Mutations within this class cause defective CFTR expression which is characterized by reduced mRNA levels, and production of little or no truncated CFTR protein. However, not all of the disease-causing alleles certain premature stop codon mutation.
Efforts have been made to suppress premature stop mutations in the ctfr gene based on the use of certain antibiotics in the aminoglycoside family, such as gentamicin, which function to suppress expression of the mutation stop codon. While the available evidence suggests that gentamicin administration can improve CFTR production and function in cells from patients having premature stop mutations, gentamicin is toxic with long term use. See http://www.personal.umich.edu/˜vplec/Gent/gent.html.
This toxicity is unfortunate as long-term therapy is required for CF patients who exhibit the following symptoms: 1) a salt imbalance resulting in secretions of abnormally thick, dehydrated mucous, 2) obstruction of digestive organs, particularly the pancreas, resulting in malnutrition, 3) clogging of lung airways with mucous, rendering them highly susceptible to bacterial infections, and 4) death from lung disease or heart failure before 30 years in about 50% of patients. See Chromosome Aberrations and Associated Diseases, http://www.urnary.edu/˜mbusch/NUR319/NUR319Chapter2notes/html.
In view of symptoms 1)-3) noted above, other palliative treatments for CF are also known. For example, deoxyribonuclease has been used to cleave DNA in sputum in order to reduce viscosity. Also, pancreatic enzymes have been administered in order to release protease, amylase and lipase into the digestive tract to compensate for the blocked ducts and tubules of the pancreas. These supplemental enzymes assist in the digestion of carbohydrates and fats which would otherwise be indigestible.
Symptom 3) leads to a host of complications ranging from bacterial infections, asthma, hemotypsis, liver dysfunction, hypoglycemia, gall bladder and kidney stones.
While all of these treatments provide some relief, the mortality rate from CF remains high over time, and, the use of aminoglycosides as stop mutation suppressors or repressors is hampered by long term toxicity. Moreover, as already noted, not all of the disease-causing alleles contain premature stop codon mutations.
Further, while gene therapies have been proposed for treating CF, this approach has not been successful as any positive results obtained are of short duration. This is largely due to the natural defenses of the lung, which defined lung tissue surfaces against allergies including dust particles, bacteria and viruses. This defense system is also effective against viral vectors used to deliver the exogenous, therapeutic genes into cells.
In view of all of these existing therapeutical shortcomings, a need exists for a new approach to transporting chloride ions, across cell membranes, whereby use of toxic repressors for the ctfr gene as well as unreliable gene therapies might be avoided, and which may be used compatibility in conjunction with other existing CF therapies to provide a single comprehensive treatment for CF.